Developing apparatus using elastic blade

Abstract: A developing apparatus includes a developer carrying member (8) for carrying a developer (6); an elastic blade (26) pressed to said developer carrying member to regulate a layer of the developer formed on said developer carrying member, said blade comprising an electrically conductive layer (26b) and a high resistance layer (26c) at the developer carrying member side of the conductive layer; electric field generating means (19) for forming an oscillating electric field between said image bearing member and the conductive layer; wherein a maximum intensity of the electric field provided by said electric field generating means is not less than 10⁶ V/m.

Description: The present invention relates to a developing apparatus usable with a copying machine, printer or the like of an electrophotographic type or an electrostatic recording type. US 4,990,959 shows a one-component developing apparatus including a developer carrier disposed in opposition to an electrostatic latent image receiving member and a toner amount restricting member pressed so as to be in contact with the developer carrier. The toner amount restricting member includes a support member made of a plate spring and a soft elastic member disposed on the support member at a portion thereof to be pressed into contact with the developer carrier. A similar apparatus is known from JP-A-57094767. Referring to Figure 1, there is shown an example of an image forming apparatus of an electrophotographic type. The electrophotographic recording apparatus 1 (printer engine) is provided with a cylindrical photosensitive drum 2 as a latent image bearing member. The photosensitive drum 2 rotates in one direction about its rotational axis, during which the surface thereof is uniformly charged by a charging device 3. Thereafter, a latent image is formed thereon by an image exposure device 4. A developing device 5 comprises a hopper 7 for accommodating a developer 6 and a developing sleeve 8 (developer carrying member). It supplies the developer 6 to a latent image formed on the photosensitive drum 2 to visualize it. Adjacent the developing sleeve 8, a developing blade 9 as a developer regulating member is disposed. Between the photosensitive drum 2 and the developing sleeve 8, a bias supplying voltage source 19 is connected to supply an AC biased DC voltage to supply a proper developing bias voltage. An image on the photosensitive drum visualized by the developer 6 is transferred onto a transfer material 11 by a transfer device 10. The transfer material 11 is fed by a sheet feeding roller 12, and is supplied to the transfer device 10 in synchronism with the image on the photosensitive drum by the registration roller 13. The visualized image of the developer transferred onto the transfer material 11 by the transfer drum 10 is conveyed to a fixing device 14 with the transfer material 11, and is fixed on the transfer material 11 by heat or pressure into an image record. The residual developer 6 remaining on the photosensitive drum without being transferred onto the transfer material, is removed by a cleaning device 15. The surface of the photosensitive drum now free of the developer 6 is charged again by the charging device 3, and the above-described steps are repeated. In the above-described electrophotographic recording apparatus, in order to facilitate maintenance operation, a developing apparatus 5 is made in the form of a developing process unit, and the photosensitive drum 2, the cleaning device 15 and the charging device 3 are made in the form of a cleaning unit 16, in which the respective units are detachably mountable. In a widely used apparatus, the two units are unified into a process cartridge 17, by which the maintenance operation is easy. The developer used in the process unit or the process cartridge is magnetic one component developer in most cases, since then no carrier remains. In the case of using the magnetic one component developer, a magnet 18 is disposed in the developing sleeve in order to retain the developer on the developing sleeve 8. Recently, in order to improve the resolution of the electrophotographic image, the particle size of the toner is reduced to approx. 6 - 9 µm. In the case of the small particle size toner, the number of particles per unit volume is large as compared with the large particle size toner, and therefore, it is difficult to provide each of the toner particles with the opportunity of contact with the surface of the developing sleeve or the surface of the developing blade with the result of the difficulty in uniform charging. Particularly, in the case of the non-contact type developing device, the charge efficiency of the toner in the developing device is low, and the uniformity of the toner charging tends to be non-uniform, and therefore, reduction of the resolution of the image density or the toner scattering or the foggy background tends to occur. As for the blade mounting method, a surface of the developing blade is contacted to the developing sleeve. In this contact type, the toner is pressed to and rubbed with the developing sleeve and the developing blade, and therefore, the toner charging power is high as compared with the no-contact type, and for this reason, it is suitable for the small particle size toner. As for the material of the developing blade of the contact type, there are metal spring such as phosphor bronze, elastic rubber such as urethane or silicone rubber, which is disposed to provide a predetermined contact pressure to the developing sleeve. In order to further improve the toner charge efficiency, efforts are made to further reduce the thickness of the toner layer on the developing sleeve for the purpose of increasing the opportunity of the contact of the toner with the developing sleeve or the developing blade. The toner layer thickness can be controlled to some extent by the surface roughness of the developing sleeve, the contact pressure of the developing blade, the hardness of the developing blade or the like. However, in the case of the one component magnetic developer, it is frequently controlled by the surface roughness of the developing sleeve. The toner layer thickness decreases with decrease of the surface roughness of the developing sleeve, and therefore, the toner charging amount increases therewith. The toner charge efficiency increases with the reduction of the toner layer thickness as described above. However, if this is done, the amount of the toner supply decreases with the result of lower image density, and therefore, there is a lower limit to the thickness of the layer. With the decrease of the thickness of the toner layer, the state of rubbing between the developing sleeve and the developing blade becomes more influential with the result of easier non-uniformity of the toner application. To cope with this, the peripheral speed ratio between the photosensitive drum and the developing sleeve is increased to increase the toner supply amount, or a low hardness elastic member (not more than 70 degrees, Asker C) is used at a contact portion of the blade with the developing sleeve, thus suppressing the non-uniform application. On the other hand, if the toner charge amount is increased too much, the surface of the developing sleeve or the surface of the developing blade are coated with strongly charged toner (high charge toner) constituting a high charge layer, with the result of various problems. The high charge layer is produced through the following mechanism. The charged toner is subjected to electrostatic mirror force proportional to the amount of charge and to a distance from the developing sleeve or the developing blade. In addition, the toner deposited on the developing sleeve surface or the developing blade surface receives other various physical attraction forces. Therefore, if the high charge toner is once deposited on the developing sleeve or the developing blade, various physical attraction forces are applied including the electrostatic mirror force as a major component with the result that it is not easily removed. The high charge layer prevents contact between the newly supplied toner from contacting to the developing sleeve or the developing blade. Thus, the triboelectric charge between toner particles necessarily increases, and therefore, the amount of the oppositely charged toner (reverse toner) is relatively increased with the result of reduction of the image density or the increase of the fog in the background portion. The high charge layer reduces the charge efficiency, which causes non-uniform toner charging. As a result, if only a part of the toner on the developing sleeve is consumed for the development, the amount of charge of the toner becomes non-uniform between the consumed portion and not-consumed portion. This influences in the next image formation (ghost) in some cases. Particularly under the low humidity condition, the high charge layer covering the developing sleeve is further strongly charged with the result of non-uniform charging, non-uniform application of the toner or another improper image formation (blotch) in some cases. As described, the influence by the existence of the high charge layer on the developing sleeve is significant. Particularly in the case of small size toner particles, they are more closely deposited on the developing sleeve surface of the developing blade surface, and therefore, the problems are more significant. Japanese Patent Application Publication No. 41068/1988 proposes that an alternating voltage is applied between the developing sleeve and a magnetic blade of iron to accomplish uniform toner application on the sleeve. However, the toner is not triboelectrically charged by the friction between the blade and the toner in this publication. Japanese Patent Application Publication NO. 19145/1993 discloses that an alternating voltage is applied to a metal blade to cause fine vibration of the blade. However, this involves a problem of electrical leakage between the developing roller and the metal blade. Accordingly, it is a principal object of the present invention to provide a developing apparatus in which a high charge toner layer is not formed, so that the toner charging efficiency can be improved. It is another object of the present invention to provide a developing apparatus in which electrical leakage between a developer carrying member and an elastic blade is prevented. According to an aspect of the present invention, there is provided a developing apparatus comprising: a developer carrying member for carrying a developer; an elastic blade pressed to the developer carrying member to regulate a layer of the developer formed on the developer carrying member, the blade comprising an electrically conductive layer and a high resistance layer at the developer carrying member side of the conductive layer; electric field generating means for forming an oscillating electric field between the developer carrying and the conductive layer; wherein a maximum intensity of the electric field provided by the electric field generating means is not less than 10 6 V/m. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. Figure 1 is a sectional view of an electrophotographic apparatus. Figure 2 is a sectional view of a developing apparatus according to an embodiment of the present invention. Figure 3 is a sectional view of a part of the developing apparatus of Figure 2, illustrating high charge toner layer. Figure 4 is a sectional view of a part of the developing apparatus of Figure 2 in which the behavior of the high charge toner layer at a downstream portion is illustrated with elapse of time. Figure 5 is a sectional view of a developing apparatus according to another embodiment of the present invention. Figure 6 is a sectional view of a part of a developing apparatus according to a further embodiment of the present invention. Figure 7 is a sectional view of a developing apparatus according to a further embodiment of the present invention. Figure 8 is a circuit diagram of an equivalent circuit of a developer regulating portion of the developing apparatus of Figure 7. Figure 9 is a sectional view of an image forming apparatus provided with a developing apparatus according to a comparative arrangement. Figure 10 is a sectional view of an image forming apparatus provided with a developing apparatus according to a comparative arrangement. Figure 11 is a sectional view of a process cartridge containing a developing apparatus according to an embodiment of the present invention. Referring to Figure 2, there is shown a developing apparatus according to an embodiment of the present invention. A developer regulating member in the form of a developing blade 26 comprises a supporting metal plate 26a fixed to a hopper 7, an electrically conductive rubber member 26b (carbon-dispersed EPDM) bonded to the supporting metal plate at the end thereof, a high resistance layer 26c of urethane resin material having a thickness of approx. 50 µm covering the surface of the electrically conductive rubber member. A free end of the conductive rubber 26b is counterdirectionally contacted to the developing sleeve 8. To the conductive rubber 26b, a bias voltage source 27 is connected through the supporting metal plate 26a, so that a predetermined bias voltage is applied thereto. The high resistance layer 26c is provided to prevent leakage between the developing blade 26 and the developing sleeve 8. The rubbing portion between the developing sleeve 8 and the developing blade 26 is approx. 2 mm, and the contact pressure is approx. 30 g/cm, and the peripheral speed of the developing sleeve 8 is approx. 94 mm/sec. The developing sleeve 8 is connected with a developing bias voltage source 19 to apply a predetermined developing bias voltage between the developing sleeve 8 and the photosensitive drum 2. Therefore, the electric field generating means in this embodiment comprises a developing blade 26, a developing sleeve 8 and a blade bias voltage source 27 and a developing bias voltage source 19. In this embodiment, the toner used is a toner having a particle size of approx. 6 µm and chargeable to the negative polarity. The applied developing bias voltage is as follows: DC voltage Vdc = -400 V Alternating voltage: Rectangular wave Amplitude Vac = 1.7 kVpp Frequency f = 1800 Hz The blade bias voltage applied to the blade was Vdc = 0 V, and no AC voltage was applied. In other words, the developing blade was electrically grounded. In the case of powder toner, the charge amount of the individual toner particles is mostly not more than 10 -3 c, and the electrostatic mirror force of the individual toner particles is mostly not more than 10 -7 N. Therefore, the maximum magnetic intensity of the AC electric field is not less than 10 6 V/m. If it is not less than 10 6 V/m, the high charge toner layer can be removed from the developing sleeve only the force provided by the electric field. However, if the physical attraction forces due to the surface tension water deposited on the toner, the magnetic confining force or the like is strong, the maximum intensity is preferably not less than 10 7 ohm/cm. If it increases 10 8 V/m, the electric leakage may occur, and therefore, 10 8 V/m or lower is preferable. In this embodiment, as described in the foregoing, the regulating bias voltage is applied between the sleeve and the blade, so that attraction force to the developing blade is produced by the electric potential difference between the developing sleeve and the developing blade, by which the developing blade is vibrated. Therefore, the force provided by the above-described electric field and the vibration of the developing blade are applied to the toner. This is one of the features of this embodiment. The voltage applied between the developing sleeve and the developing blade as described above, is -1200 V and +400 V, with the reference of the developing blade. The urethane resin high resistance layer has a thickness of approx. 50 µm, and the specific dielectric constant thereof is approx. 3. The thickness of the toner layer is approx. 100 µm, and the dielectric constant thereof is approx. 1. If these are reasonably assumed, the maximum electric field is 1x10 7 V/m in the direction of removing the toner from the developing sleeve in the high charge toner layer. The range of contact pressure variation is approx. 1.5 g/cm. It has been confirmed that in this embodiment the high charge toner layer can be removed from the developing sleeve if the range of variation of the contact pressure is approx. 10 % of the contact pressure. In this embodiment, the contact pressure of the developing blade is approx. 30 g/cm, and therefore, the range of variation of the contact pressure does not satisfy this. However, the high charge layer can be further easily removed as compared with the first embodiment. The reason for this is considered as follows. A synagism effects are provided by the force in the peripheral direction of the developing sleeve by the vibration of the developing blade and the force by the electric field substantially perpendicular to the surface of the developing sleeve, is provided. Referring to Figure 6, the behavior will be further described. By the vibration of the developing blade 26 due to the potential difference between the developing sleeve and the developing blade, the high charge layer is given the peripheral force with respect to the developing sleeve 8 surface (hatched arrow in the Figure), so that the physical attraction force is reduced to made removal from the developing sleeve easier. When the electric field is applied perpendicularly to the developing sleeve 26, the high charge toner layer is easily removed from the developing sleeve by the Coulomb's force. The easy removal of the high charge toner layer is caused by the forces in the different directions. If there is no force due to the variation of the contact pressure of the like, and only the force due to the electric field is applied, it is difficult to remove the high charge toner layer from the developing sleeve. It is desirable that the direction of the electric field is to remove the toner from the developing sleeve. In this embodiment, the force applied between the developing blade and the developing sleeve alternates, and therefore, the high charge toner layer once removed moves between the developing blade and the developing sleeve, and therefore, they are stirred. In the region downstream of the contact position, as shown in Figure 4 with elapse of time (from the right to the left), the toner reciprocates so that strong stirring effect can be provided. Thus, the charge non-uniformity can be almost removed. To enhance the stirring effects, an alternating electric field is applied between the developing blade and the developing sleeve. In addition, simultaneously with the removal of the charge non-uniformity, the toner application non-uniformity due to foreign matters between the developing blade and the developing sleeve can be removed to the extent of practically no problem. The experiments of this embodiment will be described. In this experiments, a high charge toner layer of normal black toner is formed on the developing sleeve, and the toner other than the toner in the high charge toner layer is removed, and another toner having another color but substantially the same properties is supplied. The degree of the high charge toner removing effect has been investigated with a parameter of an amplitude Vac of the developing bias. The developing bias voltage was: DC voltage Vdc = -400 V Alternating voltage: Rectangular wave Frequency f = 1800 Hz Blade bias: DC voltage Vd = 400 V As a result, the lower limit of the high charge toner layer removing effect has been Vac = 1000 V. The voltage applies between the developing sleeve and the developing blade is ±500 V with the reference of the potential of the developing blade. If this other conditions are the same as the above-described embodiment, the range of variation of the contact pressure has been measured as approx. 0.38 g/cm. The maximum electric field at the rubbing portion is approx. 4.2x10 6 V/m. The lower limit of the high charge toner layer formation prevention has been Vac = 500 V. The voltage applied between the developing sleeve and the developing blade is ±250 V with the reference of the potential of the developing blade. If the other conditions are the same as the above-mentioned embodiment, the range of variation of the contact pressure has been approx. 0.095 g/cm, and the maximum electric field in the rubbing portion has been 2.1x10 6 . When the above-described electric field is applied, the variation of the contact pressure for preventing high charge toner layer formation and the high charge toner layer removal may be quite low, as compared with the case when the electric field is not applied. The proper range of the contact pressure variation is dependent upon the rotational speed of the developing sleeve or the target toner layer thickness or the like. However, if the contact pressure is 10 - 300 g/cm, the maximum electric field of not less than 10 6 V/m is enough to provide the prevention effect of the high charge toner layer formation even if the variation range of the contact pressure is 0.3 % of the contact pressure. In order to provide the high charge toner layer removal effects with these conditions, the contact pressure variation range is not less than 1 % of the contact pressure. Referring to Figure 5, a second embodiment of the present invention will be described. In this embodiment, the developer regulating member is in the form of a developing blade 28 comprises a non-magnetic SUS metal plate 28a having a thickness of 0.1 mm fixed to the hopper 7, a polyamide resin high resistance layer 28b having a thickness of approx. 40 µm and covering the surface of the metal plate, and urethane rubber 28c having a thickness of approx. 0.4 mm bonded to such a portion of the free end of the high resistance layer as is in rubbing contact with the developing sleeve 8. The developing blade 28 is contacted codirectionally with the rotational movement of the developing sleeve. In this embodiment, the rubbing portion between the developing sleeve 8 and the developing blade 28 is approx. 2.5 mm and the contact pressure per unit longitudinal length is approx. 30 g/cm, and the peripheral speed of the developing sleeve 8 is approx. 94 mm/sec. The non-magnetic SUS metal plate 28a is connected with a blade bias voltage 29 so that a predetermined bias voltage can be applied. The developing sleeve 8 is connected with a developing bias voltage source 19 to apply a predetermined developing bias voltage between the developing sleeve 8 and the photosensitive drum 2. The polyamide resin high resistance layer 28b is provided to prevent electrical leakage between the developing blade 28 and the developing sleeve 8. In this embodiment, a negatively chargeable toner is used as in the foregoing embodiment. The developing bias voltage was: DC voltage Vdc = -400 V AC voltage: Rectangular wave Amplitude Vac = 1.6 kVpp Frequency f = 1800 Hz The blade bias voltage was: DC voltage Vdc = 0 V AC voltage: Sine wave Amplitude Vac = 1.0 kVpp Frequency f = 500 Hz In this embodiment, the urethane rubber has a thickness of approx. 400 µm, the specific dielectric constant is approx. 3. The nylon resin high resistance layer has a thickness of approx. 40 µm and a specific dielectric constant of approx. 4. The toner layer has a thickness of approx. 60 µm and a specific dielectric constant of approx. 1. The voltage is 1700 V at the maximum with the reference of the potential of the developing blade, and therefore, the maximum strength of the electric field is approx. 8.4x10 6 V/m, and the contact pressure variation range in the rubbing portion is approx. 1.0 g/cm. Under these conditions, the electric field applied between the developing blade and the developing sleeve is quite complicated, but it has been confirmed that the high charge toner layer is sufficiently removed. Similarly to the foregoing embodiment, a developing bias voltage is applied between the sleeve and the blade, and the attraction force is applied to the developing blade due to the potential difference between the developing sleeve and the developing blade with the result of vibration of the developing blade. Therefore, the toner is subjected to the force due to the electric field and the force due to the vibration of the developing blade. With the electric field and the vibration capable of removing the high charge toner layer, the waveform of the electric field may be any type. For example, the blade bias voltage may be in the form of a rectangular wave for sew teeth wave. The vibration of the developing blade is provided by the potential difference from that of the faced developing sleeve, and therefore, the electric field applied between the developing blade and the developing sleeve may be one way (the direction is not reversed). As will be understood from the foregoing description, in this embodiment, at or adjacent the contact portion between the developing sleeve and the developing blade, the fine vibration is imparted to the developing blade to remove the high charge layer, and this is important. In addition, by the application of the electric field, the fine vibration is produced in the developing blade, and the concurrently occurring electric field between the developing sleeve and the developing blade, the high charge layer is easily removed. The configuration, structure, the material or the like of the developing blade or the developing sleeve are not limited as long as the easy removal is accomplished. For example, as shown in Figure 6, an electrode 30 may be provided inside the developing sleeve 8, and an oscillating voltage is applied between the electrode and the developing blade 28 with the developing sleeve 8 interposed therebetween. In this case, the developing sleeve may be of phenol resin, epoxy resin or another non-metal or non-conductive material. As for the developing process, it may be a contact process or non-contact process. Furthermore, the developing bias voltage may be DC voltage or an AC voltage. Referring to Figure 7, another embodiment of the present invention will be described. The developer regulating blade is in the form of an elastic blade 26 comprises an electrically conductive rubber layer 26b functioning as an electrode and supported on the supporting metal plate 26a by bonding, and a resin high resistance layer 26c covering such a surface thereof close to the developing sleeve 9. In this embodiment, the conductive rubber layer 26b is of carbon-dispersed EPDM. The thickness of the resin high resistance layer 26c applied on the surface thereof has a thickness of approx. 50 µm. The blade 26 is in the form of a resin high resistance layer 26c and is contacted to the developing sleeve 9, and the contact therebetween is counter-directional with respect to the rotational direction of the developing sleeve 9. In this embodiment, the contact portion between the blade 26 and the developing sleeve 9 has a dimension of approx. 2 mm, and the contact pressure therebetween per unit length is approx. 30 g/cm. The peripheral speed of the developing sleeve 9 is approx. 94 mm/sec. The conductive rubber layer 26b is supplied with a predetermined bias voltage by way of the supporting metal plate 26a from a voltage source 27 connected to the supporting metal 26a. The resin high resistance layer 26c is provided to prevent the electrical leakage between the blade 26 and the developing sleeve 9. The developing sleeve 9 is connected with a voltage source 19 so as to be supplied with a predetermined developing bias voltage between the photosensitive drum 2. In this embodiment, the use is made with a negatively chargeable one component magnetic insulative toner 6 having a particle size of approx. 6 µm. The developing bias voltage applied to the developing sleeve 9 was: DC voltage Vdc = -400 V AC voltage: Rectangular wave Amplitude Vac = 1.6 kVpp Frequency f = 1800 Hz In order to investigate relationships among a bias voltage applied to the blade 26, thickness and dielectric constant of the high resistance layer 26c of the blade 26 and a degree of improvement in the image quality, the following experiments have been carried out. The gap D between the blade 26 and the developing sleeve 9 is 100 µm. Experiment 1 As the high resistance layer 26c of the blade 26, a polyethylene sheet having a dielectric constant εr = 2 was used with different thicknesses t = 10, 20, 50, 100, 500 and 1000 µm. Different DC voltages Vdc = -1200, -1150, -1100, -1050, -1000, -900, -800 and -600 V were applied to the blade 26. With these conditions the developing operations were carried out, and the image qualities were evaluated. It is considered that the high resistance layer 26c in the portion where the blade 26 and the developing sleeve 9 are faced is a parallel plate having an electrostatic capacity (capacitance) CI and that a gap D between the blade 26 and the developing sleeve 9 is in the form of a parallel plate having an electrostatic capacity of C2. Then, an equivalent circuit is deemed as shown in Figure 8 (a series of the electrostatic capacitances C1 and C2). With the potential difference v between the blade and the developing sleeve 9, the electric field ED produced in the cap D between the blade 26 and the developing sleeve is: ED = v/(D+t/εr) [V/m] In this embodiment, the developing bias voltage applied to the developing sleeve was in the form of a rectangular wave, and the bias voltage applied to the blade 26 was a DC voltage, and therefore, the electric field ED had two levels. Table 1 shows the result of evaluation of the image quality, the strength of the electric field in the direction of removing the toner from the developing sleeve 9 of the electric field produced between the blade and the developing sleeve. Experiment 2 As the high resistance layer 26c of the blade 26, the use was made with phenol resin having a specific dielectric constant εr = 5. Similarly to Experiment 1, the developing operations were performed with different thicknesses of the high resistance layer and the blade bias voltage Vdc, and the produced images were evaluated. The results are shown in Table 2. As will be understood from Tables 1 and 2, the image quality is improved with decrease of the thickness t of the high resistance layer 26c and with increase of the blade bias voltage Vdc, and the preferable ranges of the thickness t and the bias voltage Vdc increase with increase of the specific dielectric constant of the high resistance layer 26c. As will be understood from Tables 1 and 2, the image quality is improved with increase of the strength of the electric field (calculated value) in the direction of removing the toner from the developing sleeve 9 of the electric field produced in the gap between the blade 26 and the developing sleeve 9. The results of the electric field strength are summarized as shown in Table 3 from Tables 1 and 2 on the basis of experiments 1 and 2. Electric field intensity (calculations) Image quality evaluation ≧0.9x10 6 V/m G ≧0.14x10 6 V/m F ≦0.08x10 6 V/m N From the foregoing the following results are obtained. When it is assumed that a high resistance layer 26c at the position where the blade 26 and the developing sleeve 9 are opposed is considered as a parallel plate capacitance, that the gap D between the blade 26 and the developing sleeve 9 is considered as parallel plate electrostatic capacitance, that the electrostatic capacitances are connected in series, that a potential difference between the blade 26 and the developing sleeve 9 is v. It is satisfactory that the electric field ED produced in the gap D between the blade 26 and the developing sleeve. ED = v/(D+t/εr) [V/m] is not less than 0.90x10 6 V/m, preferably not less than 1x10 6 V/m. If this is satisfied, the strength of the electric field is enough to unbind the toner to remove the high charge toner layer and the foreign matter or the like. Therefore, the non-uniformity in the toner application and the charging is eased to provide proper developing, thus permitting high quality image formation. The provision of the high resistance layer 26c of the blade 26 is effective to avoid non-uniform electric discharge attributable to the leakage of the blade bias voltage to the developing sleeve 9. A further embodiment will be described. This embodiment is similar to that of Figure 7, but a DC bias is used for the developing bias voltage applied to the developing sleeve 9, and a DC biased AC voltage was used as the bias voltage applied to an elastic blade 26. The blade 26 comprises an electrically conductive rubber layer 26b of carbon-dispersed EPDM bonded to the supporting metal plate 26a, and a high resistance layer 26c of polyamide resin (εr = 3) having a thickness of 20 µm on the surface of the conductive rubber layer 26b. A negatively chargeable toner having a particle size of approx. 6 µm was used, and the developing bias voltage applied to the developing sleeve as: DC voltage Vdc = -400 V The blade bias voltage to the blade was: DC voltage Vdc = -400 V AC voltage: Rectangular wave Amplitude Vac = 1.6 kVpp Frequency f = 1800 Hz The gap D between the blade and the developing sleeve is 100 µm, and the strength of the electric field in the direction of removing the toner from the developing sleeve 9 to the blade 26 is: ED = v/(D+t/εr) = {700-(-400)}/{100x10 6 +20x10 -6 /3} = 10x10 6 [V/m] This satisfies the condition of not less than 1x10 6 V/m. In this embodiment, high quality of the images were produced through proper development also in this embodiment. Figure 9 shows an image forming apparatus having a developing apparatus according to a comparative arrangement. First, the description will be made as to the problems intended to be solved by this arrangement. Generally speaking, the toner has a certain degree of particle size distribution. The fine toner particles having small diameter tend to receive too much triboelectric charge amount per unit volume, and therefore, they are accumulated on the surface of the developing sleeve by the mirror force of the fine toner to constitute a fine toner layer. This increases gradually. Therefore, the majority of the toner particles having the particle size close to the average particle size, which are intended to perform major and decisive role in the development of the latent image if it is charged to the proper level is prevented from properly performing the role by the fine toner layer with the result that the triboelectric charge provided by the friction with the surface of the developing sleeve is deteriorated, and therefore, the developing property is deteriorated. This deterioration results in the decrease of the image density, or the production of the sleeve ghost as a result of the influence of the hysteresis of the previous developed image to the next image development. In a non-contact development, when an image is reproduced with the density of not less than 600 dpi, the latent image can be more faithfully reproduced if the distance between the developing sleeve and the photosensitive drum (S-D gap) is decreased. However, the decrease of the S-D gap results in increase of fog, and leakage of the developing bias under low humidity condition. If the AC voltage component of the developing bias voltage is reduced in an attempt to prevent the leakage, the image density is decreased. This arrangement shown in Figure 9 is intended to provide an image forming apparatus in which even if fine toner layer is formed on the surface of the developing sleeve, the one component developer on the developing sleeve is properly charged, so that the resultant image is free of the sleeve ghost or another defect. As shown in Figure 9, a charging roller 108 is contacted to the photosensitive drum 107, and a bias voltage is applied from a voltage source 143 to a core metal of the charging roller, so that the surface of the photosensitive drum is uniformly charged by the charging roller 108. The photosensitive drum 107 is exposed to image light through an unshown optical system so that an electrostatic latent image is formed. The latent image formed on the photosensitive drum 107 is developed with a developer by the developing device 125A into a toner image. The developing device 125A contains one component magnetic developer in the developer container 125. The developing sleeve 131 disposed in the developer container 125, in this arrangement, comprises an aluminum sleeve functioning as an electrode and a high resistance layer 131a thereon. The high resistance layer 131a may be applied on the aluminum sleeve base through spray or dipping method using heat-curing resin such as phenol resin or the like in which carbon black, graphite or other conductive particles are dispersed. Thereafter, the material is heat-cured to provide a volume resistivity of 3x10 8 - 1x10 10 ohm.cm with the film thickness of 5 - 20 µm. During the developing operation, the developing sleeve 131 is supplied with a developing bias voltage comprising an AC component having a peak-to-peak voltage of 200 V and a frequency of 1800 Hz and a DC component of -600 V from a voltage source 130. The elastic blade 124 disposed at a position of the container 125 above the developing sleeve 131, according to this arrangement comprises electrically conductive member such as conductive rubber (carbon dispersed EPDM), phosphor bronze, stainless sheet or the like. The surface of the elastic blade 124 is not provided with an insulative layer. In this arrangement, the elastic blade 124 is supplied with an oscillating voltage in the form of a DC biased AC voltage from the voltage source as the bias voltage. In this example, the bias voltage has a DC component of -800 V. The toner image obtained through the development, is transferred onto a recording material 102 fed to the image transfer station where a transfer roller 104 is opposed to the photosensitive drum 107. Thereafter, it is fixed by an unshown fixing device. The toner remaining on the photosensitive drum 107 is removed by a cleaning blade 111a from the photosensitive drum 107 so as to be prepared for the next image forming operation. In this arrangement, the surface of the developing sleeve 131 is coated with a high resistance layer 131a, no excessive current flowing between the developing sleeve 131 and the elastic blade, even if they are partly contacted directly with each other upon the start of a new developing device or at the time of the short of the toner. According to this arrangement, the magnetic toner T1 carried on the developing sleeve 131 is triboelectrically charged by the rubbing with the surface of the developing sleeve 131 during passage through the nip formed with the elastic blade 124, and is reciprocated between the developing sleeve and the elastic blade after the passage through the nip, in accordance with the change of the strength and the direction of the electric field between the elastic blade and the developing sleeve. For this reason, the high charge toner close to the surface of the developing sleeve 131 and the low charge toner at the surface portion of the toner layer, are mixed together, so that it can be avoided that the high charge amount toner is electrostatically attracted on the surface of the developing sleeve and obstruct the triboelectric charge of the other toner. Therefore, the low density or the sleeve ghost under the low humidity condition can be decreased. In addition, it is possible to increase the charge amount or to maintain the same stably, the low density or sleeve ghost due to the moisture absorption of the toner under the high humidity condition, can be also avoided. A further arrangement will be described. One of the features of this embodiment is the use of the toner having a volume average particle size of 4 - 9 µm as one component magnetic toner T1. The selection of the particle size is because when the latent image of not less than 600 dpi is to be developed, the reproducibility of fine lines is improved by the use of the toner having the particle size not more than 9 pm, and because when the toner is manufactured through pulverizing method at preset, it is difficult to stably contain the magnetic material in the toner without increase of the cost, if the size is smaller than 4 µm. When the size of the toner particle T1 is reduced, the surface per unit volume of the toner increases, and therefore, the amount of the electric charge per unit weight increases. The high charge amount toner having small particle size is strongly attracted electrostatically on the surface of the developing sleeve by the mirror force thereof. Therefore, when the size of the toner particles is reduced, the reduction of the density under the low humidity or the sleeve ghost are worsened. Therefore, in this arrangement, the electric field applied to the toner adjacent the nip with the elastic blade 125 is increased, and the period thereof can be set to correspond to the toner property by using a DC biased AC voltage applied to the elastic blade. In this embodiment, the DC component is -900 V, and the AC component has a peak-to-peak voltage of 1500 V and a frequency of 3000 Hz. In this case, not only the force by the electric field applied to the toner T1 is increased, but also a potential difference not less than a discharge starting voltage (550 V under normal temperature and normal pressure condition) is periodically produced between the elastic blade 24 and the toner layer surface, and therefore, it is possible to charge the toner on the developing sleeve 31. Therefore, the low density image or the sleeve ghost under the low humidity condition which is a problem when fine magnetic toner T1 is used, can be significantly reduced. In addition, the density reduction due to the reduction of the triboelectric charge amount attributable to the moisture absorption of the toner under the high humidity condition can be improved. Figure 10 schematically shows an image forming apparatus provided with a developing apparatus according to a comparative arrangement. One of the features of this arrangement is the use of insulative one component non-magnetic toner T2. The developing sleeve 131 is not provided with a magnet roller. In order to supply and carry the toner T2 on the developing sleeve 131, an elastic roller 126 in the form of a sponge roller is contacted to the developing sleeve 131 at a rear position of the developing sleeve 131 in the developer container 125. In this arrangement, pigments used for the control of the resistance of the high resistance layer 131a formed on the surface of the developing sleeve 131 is of graphite. The graphite has cleavage crystalline plane, and therefore, the parting property is excellent, and therefore, the contamination of the developing sleeve with the toner which easily occurs in the case of non-magnetic toner, can be prevented. To the non-magnetic toner T2 carried on the developing sleeve 131, the electric charge is supplied to the toner T2 after passing by the elastic blade 124, by which the amount of the charge of the toner is made uniform, the density of the image is stabilized, and the sleeve ghost is reduced. Thus, the better developing operation than the prior art can be accomplished. Figure 11 shows a process cartridge using the developing apparatus of this arrangement. Generally, the process cartridge comprises a photosensitive drum and at least one of the charging means, developing means and the cleaning means in the form of a cartridge, which is detachably mountable to a main assembly of the image forming apparatus. The consumable articles represented by the photosensitive drum and the developer are formed as a unit, so that the user can easily replace it, thus accomplishing fundamentally maintenance-free image forming apparatus. No insulative layer is provided on the surface of the elastic blade 124 of the developing apparatus 125A, and a high resistance layer is provided on the surface of the developing sleeve. At one side of a casing 150 of the cartridge, a photosensitive drum 107 protected by a shutter 151 is disposed. The shutter 151 is opened when the cartridge is mounted into the image forming apparatus to provide an image transfer opening faced to the photosensitive drum 107. The image light from an image exposure apparatus outside the cartridge is introduced through the exposure opening 109 at the opposite side of the shutter 151 so as to permit exposure of the photosensitive drum 107 thereto. The developing device 125A is at the bottom of the casing 150 of the process cartridge. In the developer container 125, a developer chamber 125a with the developing sleeve 131 below the photosensitive drum 107 is formed. At a side of the developing chamber 125a, a toner container 125b is formed for containing the one component magnetic toner T1. The toner T1 in the toner container 125b is fed into the developing chamber 125a through an opening 125c from the toner container 125b by a feeding member 140, so that it is supplied and carried on the developing sleeve 131. The toner T1 carried on the developing sleeve 131 is regulated in the layer thickness thereof by the elastic blade 124 supplied with an oscillating bias voltage from an unshown voltage source. As described hereinbefore, adjacent the nip between the elastic blade 124 and the developing sleeve 131, the high charge amount toner distributed adjacent the sleeve surface of the toner layer on the developing sleeve and the low charge amounts toner distributed adjacent the outside part of the toner layer, are mixed. Therefore, uniform and stabilized charge is applied through the entire toner layer on the developing sleeve. Therefore, the electrostatic latent image formed on the photosensitive drum 7 can be properly developed without ghost or low image density. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims: A developing apparatus comprising a) a developer carrying member for carrying a developer, b) an elastic blade pressed to said developer carrying member to regulate a layer of the developer formed on said developer carrying member, said blade comprising an electrically conductive layer and a high resistance layer at the developer carrying member side of the conductive layer, and c) electric field generating means for forming an oscillating electric field between said developer carrying member and the conductive layer, d) wherein a maximum intensity of the electric field provided by said electric field generating means is not less than 10 6 V/m. An apparatus according to Claim 1, wherein the maximum intensity is not more than 10 8 V/m. An apparatus according to Claim 1, wherein the oscillating electric field has a frequency of 10 Hz - 10 KHz. An apparatus according to Claim 1, wherein the conductive layer is of metal, and the high resistance layer is of rubber. An apparatus according to Claim 1, wherein the high resistance layer is on the conductive layer. An apparatus according to Claim 1, wherein said electric field generating means applies an oscillating voltage both to said developer carrying member and the conductive layer. An apparatus according to Claim 1, wherein the developer is a one component magnetic toner. An apparatus according to Claim 1, wherein the conductive layer and the high resistance layer are of rubber material. An apparatus according to Claim 1, wherein the oscillating electric field is an alternating electric field. An apparatus according to any one of Claims 1-9, wherein said developing apparatus is in the form of a cartridge with an image bearing member for bearing an electrostatic image, and said developing apparatus is detachably mountable relative to an image forming apparatus.

Inventors: ASANO ERIKA, HASEGAWA HIROTO, KATO JUNICHI, KISU HIROKI, OGATA HIROAKI, OHKUBO MASAHARU, OHZEKI YUKIHIRO, OJIMA MASAKI, SAKAIZAWA KATSUHIRO, SAKURAI KAZUSHIGE, SATO HIROSHI, YAMAZAKI MICHIHITO, ASANO, ERIKA, HASEGAWA, HIROTO, KATO, JUNICHI, KISU, HIROKI, OGATA, HIROAKI, OHKUBO, MASAHARU, OHZEKI, YUKIHIRO, OJIMA, MASAKI, SAKAIZAWA, KATSUHIRO, SAKURAI, KAZUSHIGE, SATO, HIROSHI, YAMAZAKI, MICHIHITO, ASANO, ERIKA, C/O CANON KABUSHIKI KAISHA, HASEGAWA, HIROTO, C/O CANON KABUSHIKI KAISHA, KATO, JUNICHI, C/O CANON KABUSHIKI KAISHA, KISU, HIROKI, C/O CANON KABUSHIKI KAISHA, OGATA, HIROAKI, C/O CANON KABUSHIKI KAISHA, OHKUBO, MASAHARU, C/O CANON KABUSHIKI KAISHA, OHZEKI, YUKIHIRO, C/O CANON KABUSHIKI KAISHA, OJIMA, MASAKI, C/O CANON KABUSHIKI KAISHA, SAKAIZAWA, KATSUHIRO, C/O CANON KABUSHIKI KAISHA, SAKURAI, KAZUSHIGE, C/O CANON KABUSHIKI KAISHA, SATO, HIROSHI, C/O CANON KABUSHIKI KAISHA, YAMAZAKI, MICHIHITO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: G03G15
IPC codes: G03G15, G03G13